This invention relates to circuits for preventing arcing upon connection or disconnection of a load to a direct current source. In particular, this invention relates to such arc preventing circuits which are not connected in parallel to the point of connection of the load to the direct current source.
The prevention of contact arcing is most important when a current source may be used in an atmosphere containing combustible gases. In such applications it is also desirable to prevent component overheating from short circuiting of the current source.
Most prior art circuits for preventing arcing upon connection or disconnection of a load to a direct current source have relied on their being circuit elements in parallel with the switch or connection point at which the direct current circuit is made. Such a parallel circuit implies a more or less permanent connection between the load and the current source. Typical of such prior art circuits is that described by Passrella in U.S. Pat. No. 3,912,941. The circuit described by Passarella utilizes a transistor arranged in series with the load. The transistor is controlled by a manual switch which is connected to one pole of the current source. Current is supplied to the load when the control switch is closed turning on the transistor.
U.S. Pat. No. 3,122,659 to Krestel et al also describes a prior art circuit which contemplates a relatively permanent connection between the current source and the load. The apparatus described by Krestel uses a mechanical relay to close the circuit between the current source and the load. Arranged in series with the relay is an SCR controlled rectifier which is triggered by the closing of the mechanical relay. The circuit described by Krestel is similar to that described by Passarella in that there is contemplated a relatively permanent connection between the load and the current source.
In U.S. Pat. No. 3,075,124 Bagno describes a circuit, which in one embodiment may be used without permanent connection between the load and the current source. The circuit described in Bagno's FIG. 1 includes voltage dividing resistors arranged between the current source and the point of load connection. A transistor is connected parallel to the voltage dividing resistors and controlled by the intermediate voltage. In the FIG. 1 embodiment a capacitor is arranged in parallel with the point of load connection, but it is pointed out in the specification that the circuit may be used without this capacitor. The Bagno circuit relies on the high frequency characteristics of the transistor to obtain arc suppression. The arrangement of voltage dividing resistors, in series with the switch, for controlling the transistor tends to effectuate a rapid transistor turn-on and rapid start of full current flow to the load. The period of delay prior to the turn-on of full current, which could cause arcing if the contact was intermittent is regulated entirely by the turn-on characteristics of the transistor used. When the contacts in the Bagno circuit are disconnected, the transistor provides some resistance against high frequency arcing, by reason of its high impedance at higher frequencies, but little protection is afforded against direct current switch arcing on disconnect, especially arcing resulting from the disconnection of an inductive load.
It is therefore an object of the present invention to provide a circuit for preventing arcing on connection or disconnection of a load to direct current source.
It is a further object of the present invention to provide such circuit having a selectable time delay prior to the initiation of current flow.
It is a still further object of the present invention to provide such a circuit for use in applications where the load is not permanently connected to the current source.
It is a still further object of the invention to provide such a circuit wherein the current source is rapidly disconnected upon the interruption of forward current.
It is a still further object of the present invention to provide such a circuit which also provides protection in the event the current source is short-circuited.